This invention relates generally to controls for hydraulic power systems and more specifically to directional control valves that selectively effect automatic leveling of a bucket on the front end of a loader or similar device during movement of the boom arm to which the bucket is attached.
It is conventional practice to provide a hydraulic cylinder and separate control valve for manipulating the bucket of a front end loader and a second cylinder and companion valve for aiding in raising and lowering the boom of a loader. In the absence of any self-leveling function, it is necessary for the operator of the loader to operate both valves, one with each hand, to maintain the bucket level while raising the boom. This operation is not only difficult but also requires the strict attention of the operator. The advantages of a self-leveling system are obvious and there have been numerous types of systems on the market for many years.
One of the more common methods, such as shown in U.S. Pat. No. 3,987,920, to Parquet, is a mechanical linkage tied to the frame of the loader which tilts the bucket, maintaining it level as the boom is raised or lowered.
Another common method, which is strictly hydraulic, is illustrated in U.S. Pat. No. 3,251,277 to Stacey. In this patent the fluid displaced from the boom cylinder is directed to the bucket cylinder by actuation of the boom spool alone. This type of system requires a matching of volumes so that the volume displacement from the boom cylinder will extend the bucket cylinder the precise distance to hold the bucket level as the boom is raised. This type of system is more expensive and bulky since it requires an unduly large bucket cylinder.
Another method is illustrated in U.S. Pat. No. 3,811,587 to Seaberg which utilizes a pair of hydraulic motors mechanically tied together with the larger motor located in the boom cylinder circuit while the smaller motor is located in the bucket circuit. As flow passes through the boom circuit, a proportionally smaller flow is forced through the bucket circuit.
Another self-leveling system is shown in U.S. Pat. No. 3,563,137 to Graber wherein the flow exiting the boom cylinder passes through a flow divider, dumping a portion to drain while directing the remaining portion to the bucket cylinder to maintain a level condition while raising the boom.
In the last-mentioned patent, the excess oil is removed from the self-leveling circuit by a proportional flow divider dumping to drain, however, such a system can only be used in a conventional parallel circuit as distinguished from a series type circuit of the present invention. A parallel circuit, as illustrated in the last-mentioned patent, provides a source of pump pressure to each valve spool in a parallel path. A series system such as U.S. Pat. No. 3,251,277 to Stacey provides a pump power passage in series through the particular valve in the system. In a series type valve, if an upstream valve is moved to an operative position, there is no pump pressure to the remaining downstream valves since the power passages are in series.
Series type valves are normally not adaptable to a self-leveling function with the exception of the last-mentioned patent to Stacey, and applicant's pending U.S. applications Ser. No. 244,831, now U.S. Pat. No. 4,408,518 and Ser. No. 464,071.
In U.S. Pat. No. 4,408,518, a flow divider is utilized in the boom discharge flow path with a portion going to the bucket cylinder while the remaining flow passes back to the valve to the boom return motor port. This last-mentioned flow through the boom return port provides downstream oil for the bucket cylinder if the operator attempts to override the self-leveling function while the boom cylinder is moving. The invention in U.S. Pat. No. 4,408,518 gives a downstream valve in a series circuit the added capacity of functioning during the movement of the blocking upstream valve; however, under certain conditions the override function of the bucket will not work.
In a three spool series circuit with the boom valve spool in self-level raise; the bucket spool in curl override; and a third spool pulled (which is downstream of the boom and bucket spool), the bucket will dump at the self-level rate instead of curling as commanded.
In applicant's second-mentioned application (Ser. No. 464,071) instead of dumping the return oil from the rod end of the bucket cylinder directly to tank, the return oil from the bucket cylinder is dumped back into the boom return motor port, thereby preventing the rod end of the bucket cylinder from dumping to tank. This insures that any increased pressure from the third downstream valve will act on the back side of the unloading spool, keeping it closed and allowing proper function of the bucket cylinder in the curl mode.